This invention relates to a process of forming an electrical interconnection pattern on a substrate and, more particularly, to a process of forming a trench or recessed pattern in an unfired ceramic substrate, which upon filling with a conductive paste and sintering will result in a ceramic package having an inlaid electrical interconnection pattern thereon.
The advantages of multilayer ceramic (MLC) substrate package are well known. The MLC typically composed of 17 to 32 layers of molybdenum-metallized alumina with a total thickness of 4 to 5 mm can typically accommodate more than one hundred semiconductor integrated circuit chips. The MLC substrate contains signal planes for inter- and intrachip connections and for routing conductors to input/output (I/O) for external connections. Layers are provided for achieving the proper electrical performance characteristics. MLC improves the circuit density, reliability and performance over those of prior art packages and compliments the advances made in very large scale and ultra large scale integrated semiconductor technology.
The basic process of manufacturing the MLC substrates consists of (1) slurry preparation by combining such raw materials as aluminum oxide, glass powders, an organic binder and a plasticizer; (2) casting the slurry into a uniform thickness (typically 0.2 mm thickness) unfired or "green" ceramic sheet tape followed by cutting the cast tape into square blanks; (3) personalization consisting of, first, punching interlevel via holes in the four corners of the sheets for precise registration of the sheets later on and, second, metallization of the sheets using a conductor paste consisting typically of molybdenum powder uniformly dispersed in a resin and solvent mixture in accordance with a desired interconnection pattern; (4) stacking and lamination which involves first stacking a desired combination of personalized green sheets by using the four-corner location holes to ensure accurate layer-to-layer alignment and then laminating to fuse the layers into a monolithic unit by the application of heat (at about 75.degree. C.) and pressure (up to about 25 MPa); and (5) sintering the laminated substrate on support plates in a kiln in a reducing atmosphere at a high temperature in the range of 1250.degree. C. to 1500.degree. C. to volatilize the organic materials and form a densified and damage-resistant MLC substrate having a firmly bonded metallurgical interconnection pattern.
The most widely used ceramic green sheet metallization process consists of screening a conductive paste on the ceramic green sheet. In this process which is disclosed in U.S. Pat. No. 4,109,377, issued to L. A. Blazick and L. F. Miller and assigned to IBM Corporation, the assignee of the present invention, a metal mask having a desired pattern of openings (i.e. a stencil) is held in close proximity or in direct contact with the green sheet and the conductive paste is extruded through the open spaces of the mask by conventional doctor blading techniques to form a conductor pattern protruding from the surface of the green sheet. A fundamental disadvantage of this process is that when the ceramic sheets are stacked together, due to the protruding nature of the conductor patterns on each sheet, a surface bulge is formed in the MLC substrate. Lamination to flatten out the bulge will cause spreading and distortion of the patterns leading to limitations on line width and cross section, spacing between lines, number of sheets that can be laminated etc. Limitations on line width and cross section translate into limitations on line resistivity since resistivity increases as the cross sectional area of the line decreases. This, in turn, means that the speed of signal propagation is decreased. A second disadvantage of the screening technique is that since the conductor patterns are formed by using a self-supporting stencil there is a limit on how narrow and close the openings therein can be formed without destroying the rigidity of the stencil which is necessary to extrude the conductor paste therethrough. Attendant to this is the problem of cleaning the stencil after each use since the finer the openings the more difficult it is to clean. These limitations amount to dimensions of the order of 25 .mu.m and a resistivity of the order of 0.5 ohm-cm.
To overcome the limitations with the above conductor screen-on process a second approach has been used in which groove or trench patterns are first formed on the surface portion of the green sheet which are then filled with the conductor paste. In this manner the conductor patterns are fully recessed or inlaid in the green sheet thereby eliminating the surface protuberance.
One way to form the groove or trench patterns is by means of a technique which is a variation of the intaglio printing in which the desired pattern that is to be inlaid in the green sheet is first formed as a positive embossment pattern on a surface (i.e. the pattern is formed on a master tool as a protrusion) and then the positive image is branded or pushed into the green sheet material leaving behind a recessed pattern. However, since the green sheet is not sufficiently soft and flexible to yield to the embossing technique and, in fact, contains a high loading of solid material which is put in to control the shrinkage of the green sheet during the sintering stage it is not practical to mechanically push a pattern into the green sheet.
Another way of forming a recessed pattern in the green sheet which is disclosed in U.S. Pat. No. 3,770,529 issued to L.S. Anderson and assigned to the present assignee is to expose the green sheet through a reflective mask having a predetermined pattern of apertures therein arranged in close proximity with the green sheet to laser radiation, thereby thermally machining the green sheet material. This approach although achieves much finer line definition and allows closer spacing of adjacent lines, necessitates a high powered laser system which is not only rather expensive, but also requires careful control.
By far the most impressive method of forming a groove or trench pattern in the green sheet is by the non-contact electron beam (e-beam) technique disclosed in U.S. Pat. No. 3.956,052 issued to W. W. Koste and E. N. Urfer and assigned to the present assignee. In this method the green sheet is laminated with a thin organic mask material and a high energy e-beam is employed to define a predetermined pattern of openings extending through the organic material and selectively into the green sheet. The e-beam technique while providing a well-defined recessed pattern in the green sheet, which can be subsequently filled with a conductor paste obtaining a highly reliable MLC package, suffers from the requirement that a e-beam pattern generator be used. Such a generator requires high vacuum tooling, sophisticated e-beam optics and custom software to control, all of which render this technique prohibitively expensive.
The present invention obviates the disadvantages of the prior art while preserving their advantages.
It is a principal object of this invention to provide a simple method of forming an inlay pattern of conductive paste on a green sheet.
Another object of the invention is to form a recessed conductive paste interconnection pattern on a ceramic green sheet such that upon stacking, laminating and sintering a plurality of such sheets the resulting package is free of surface bulge.
A further object of the invention is to provide a fully recessed conductive interconnection pattern on a ceramic green sheet of improved line definition and quality which is particularly suitable for fabricating high interconnection density multilayer ceramic packages.
Yet another object of the invention is to provide a method of forming inlaid patterns of conductive paste on a ceramic green sheet which eliminates the need for a rigid, self-supporting mask.